In the design of radio frequency (RF) chips, in order to obtain desired chip functions, it is needed sometimes to integrate silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) having different operating voltages or different characteristic frequencies (namely, cutoff frequency) into one chip. For example, on an RF transceiver chip, a power amplifier needs a high-breakdown voltage SiGe HBT to satisfy the demand for a high-power output, while a low-noise amplifier needs a standard or high-speed SiGe HBT to provide a low noise factor.
Currently adopted practice for addressing the issue of integrating SiGe HBTs having different operating voltages into one chip is to form collector regions having different doping concentrations during the manufacture of the chip, so as to achieve SiGe HBTs with different breakdown voltages, and hence provide different operating voltages. Moreover, this practice is also adopted to achieve SiGe HBTs with different characteristic frequencies by forming collector regions having different doping concentrations.
This practice requires different lithographic masks for the SiGe HBTs having different breakdown voltages to perform different ion implantation processes so as to obtain different doping concentrations in the collector regions. In the above practice, how many different operating voltages the SiGe HBTs have, how many times the lithographic process and ion-implantation process are performed, thus leading to an increased complexity in manufacturing process and a high cost.